Non-saturating transistor trigger circuits



Filed June 29. 1954 March 31, 1959l J. G. LINVILL. ETAL 2,880,330

NON-SATURATING TRANSISTOR TRIGGER CIRCUITS 2 Sheets-Sheet l v WENTORS1Q. L. WALLACE JR.

BYHGM7 C.

ATTORNEY March 31, 1959 J. G. LlNvlLL. ET A1. 2,880,330

NON-SATURATING TRANSISTOR TRIGGER CIRCUITS Filed June 29, 1954 2.Sheets-Sheet 2 .fr/'ii' 35 36 VU V L L s4 46\=ji 3/ 32 T ISL 4 av 38 37T4o ATTORNEY United States Patent O NON-SATURATING TRANSISTOR TRIGGERCIRCUITS. j.

John G. Linvill, Whippany, andRobert L. Wallace, Jr., V`Ilanlield, NJ.,assignors to'Bell Telephone" Laboratories, Incorporated, New York, NX.,a corporation of NewYork Application June 29, 195.4, SerialNo. 440,052`s claims. (ci. srsas);

This invention relates to transistor trigger circuits. Its principal`object is to increase the speed' of operation 'of such a circuit. Arelated object is to render the end conditions of the oscillations 'ofisuch" a circuit independent of the transistor or transistors employed.

In general, the action of a transistor trigger' circuit comprises arapid cumulative swing from one extreme of current and potentialconditions to "another," being limited at one end of the swing bycut-off andxat the other end by saturation. In the course of .thisswing, or switching transient as it is sometimes termed, the transistorbehaves to a large extent as a linear amplifier and can 'respond withrapidity to the inuences which cause this action; but' at thesaturation-limited extreme of its swing the transistor becomes sluggish,Thatf'is to say, once the transistor has"reached. its saturationcondition, time is required to recondition it for linear operation andso for` the following swing. It has been found that the sluggishness ofthe saturated,` transistor is a principal factor in determining thespeed of operation of the trigger circuit as a whole."

The invention provides a remedy" for this situation. By the inclusion inthe circuit of a "threshhold-responsive device'suitably connected andsuitably adjusted to change its condition at a potential which is short,of the transistor saturation potential, the transistor is permitted toremain continually in its linear operation condition, and the switchingtransient is now terminated,` not by the alteration of the transistorconditions, but by the alteration of the response of the thresholddevice.l

Furthermore, the employment of certain suitably selectedconstant-voltage elements in conjunctionwith the threshold-responsivedevice renders the magnitudes of the output currents and voltages atwhich the switching transient terminates virtually independent of thetransistor.

While the threshold device or the constant-voltage element may have anystructural form provided it has the desired characteristics, it turnsout that a semiconductor P-N junction alloy' diode, as describedbyfPear'- son and Sawyer in the Proceedings ofi the Institute of RadioEngineers for November 195.2 (volume 40, `page 348) can be adjusted toserve either purpose. For the purpose of the threshold-responsive deviceit is adjusted to shift from its high-resistance condition to itsbrokendown condition in the course of the switching transient. To serveas a constant-voltage device, it is adjusted, on the other hand, toremain continuously in its brokendovvn condition.

Many transistor trigger circuits are known. Among these one of the moreimportant is the Eccles-Jordan circuit comprising a pair of transistors,the collector electrode of each being cross-coupled to the baseelectrode of the other. With conductive cross-coupling such a circuitcan be adjusted to be bistable. Furthermore, its upper operatingfrequency is limited by the sluggishness of one of the transistors orthe other at thev saturation end of its wing.

-In accordance with the invention ina preferred form Trigger circuitsare idents# Mss, 35,195?

f. ICC

a pair of oppositely poled threshold devices, such as thebreakdown'diodes" described by Pearson and'Sawyer 'in th'evaforementioned publication,interconnect the Vtwo collector electrodes.Each of them is proportioned,`"in the fashion'described by Pearson andSawyer, to present a low forward direction resistance and, in thereverse direction, to presenta high resistance throughout a' pre:assigned voltage range and 'a very' low variational resistance forreverse voltages beyond-this range. The range is so selected thatthe-'breakdown of onel of the diodes takes place when the voltage fromone "of the' collector electrodes tothe other is less,` byv a'safe'margin, than the full collectorlto-collector voltageswing which'wouldotherwise take place. lSimilarly the other diode" breaks down when thelcollector-to-collector vo1tage'drop*is"`f the same'magnitude and ot`opposite sign. Thus, on each swing, one or the other of Ithe diodes'limitsthe voltage excursion of the collector electrode of one`transistor to a voltage which is'short of saturation. r`Incidentally,`it acts at the same time to limit thel voltage'excursion` of theemitter electrodeof the other transistor to avoltage which is short ofcut-olf. It follows that, when the switching transient is completed,bothtransistors remain iny their linear operation conditions yand are readyto' undergo another switching transient without delay."

` With A.'C. coupling in one orv both of the cross-linking paths fromcollector to base,such a circuit becomes monostable or astable and maybe operated in one or the'otherof nthese two fashions by minor potentialadjustments. Breakdown diodes may ls'obe advantageously employed withsuch circuits: to hold the transistors in their linearoperatingconditions and so accelerate the inception ofY the switchingtransient.also known which employ a single transistor instead of two.' An exampleofsuch acircuit is the blocking oscillator in"which the collector isinductively coupled to the emitter or, alternatively, and with a changeof polarity rof the coupling, to the base. The upper limiting operatingfrequency of such circuitsas they arepresently known is to a'largeextent determined by the' sluggishness manifested by the transistor whenit has been driven to saturation. A breakdown diodeor other thresholddevice may advantageously be associated with such a'circuit to restrictthe extremeA of collector voltage swing to`a value short of saturationand so to hold the transistor continuously in its linear operatingcondition.

"While the invention is applicable to transistors in general it is ofspecial importance in'connection with junction transistors-for whichsaturation sluggishness has been "a more'serious matterthan it has beenfor point contact transistors.

` The invention will be fully apprehended from the following detaileddescription of preferred'illustrative embodiments thereof, taken inconnection with the appended drawings,`in which: Y'

Fig. l is a schematic circuit diagram showing the application'cf thebreakdown diodes of the invention to a'two-transistor bistable triggercircuit of the Eccles- Jordan variety;

Fig. 1A is a simplified equivalent circuit referred to inthe explanation'of the operation of the circuit of Fig.'1; Figs. 2A and 2B show asingle breakdown diode and its voltage-current characteristic; Figs. 3Aand 3B show an oppositely poled pair of breakdown diodes and theircurrent-voltage characteristic;

Fig. s is a schematic crcuttdiesranr shorties the applieationof abreakdown diode to a single-transistor `trigger circuit.

` 1 2, are preferably, though not necessarily, of the junction type asdistinguished from the point contact type.

Resistorsl 5, 6, interconnect the respective collector electrodes withone terminal of an operating potential source 4 and resistors 7, 8,interconnect the emitter electrodes of the transistors 1, 2, with theother terminal of the same potential-source 4, which may be grounded.The resistors 7, 8, are shunted by condensers 9, 10, for a reasonexplained below. The base electrode of the first transistor 1 ismaintained at the proper operating potential by connection to anappropriate point of a voltage divider comprising a resistor 11, a'ldiode 13 whose function will be described'below, and the resistor 6.The base electrode of the right-hand transistor 2 is similarly connectedfor the same purpose to an appropriate point of a voltage divider'comprising a resistor 12, a diode y14 and the resistor 5. The collectorof each transistor is cross-coupled to the base electrode of the othertransistor by way of diodes 13, 14, and. these diodes may be by-passedby condensers 15, 16.

In accordance with the invention an additional path interconnects thecollector electrodes of the transistors 1, 2, and comprises at least onethreshold device such as a breakdown diode, and preferably two suchdevices 17, 18, oppositely poled. The terminal 19 which is common tothese two diodes 17, 18, serves as a convenient input point at whichtrigger pulses derived, for example from a pulse source 20, may beapplied to the circuit. The

resistors 7, 8, are paralleled with condensers 9, 10,

which, by virtue of their storage of energy in either of the two stablestates, provide a memory. The presence of the energy stored in thememory elements, in cornbination with the functioning of diodes 17, 18,causes the circuit to assume alternate stable states as successivepulses are applied at the point 19.

In examining the operation of the system, suppose that the left-handtransistor 1 is in its low conduction condition and the right-handtransistor 2 is in the high conduction condition, and that a pulse hasbeen applied to the trigger pulse input point 19 of a character toinitiate a switching transient; i.e., to increase conduction of theleft-hand transistor 1 and reduce conduction in the righthand transistorZ. This state of affairs proceeds to increase cumulatively in well-knownfashion; and were it not for the breakdown diodes 17, 18, wouldultimately drive the right-hand transistor 2 to cut-off and the lefthandtransistor 1 to saturation. At these points amplification by thetransistors, and therefore regeneration in the system as a whole, ceaseso that the switching transient is arrested. However, as describedabove, if the switching transient is permitted to process this far, theinitiation of another transient in opposite sense is impeded by thesluggishness of the saturated transistor.

But by virtue of the inclusion of the breakdown diodes 17, 18, after thetransient has proceeded to such a point that the right-hand transistor 2is approaching cut-olic and the left-hand transistor 1 is approachingsaturation the difference between the potential of the left-handcollector electrode, which is still falling, and the potential of theright-hand collector electrode, which isstill rising, reaches amagnitude Vm such that the diodel 17 breaks down by reason of itsinherent characteristics. It holds its breakdown voltage withsubstantially no change despite any tendency on the part of the externalcircuit to alter it. In the broken-down condition the variationalresistance of the diode 17 is very small, so that the series combinationof this variational resistance with the forward resistance of the diode18 constitutes a virtual short-circuit connection between the collectorsof the two transistors 1, 2, which shortly and firmly arrests theincrease of the potential difference between them. Thus, the switchingtransient has been fully terminated without driving the right-handtransistor 2 to cut-off, or the left-hand transistor 1 to saturation.

Once the switching transient has been arrested, reactive energy storedin the condensers 9, 10, settles to the steady value which characterizesthe stable state in which transistor 1 conducts more heavily thantransistor 2 and the system is completely at rest and is ready withoutfurther delay for the initiation of a new switching transient in theopposite direction. This new switching transient, like its predecessor,may be initiated by the application of a pulse to the terminal 19.

The following consideration of the circuit in this quiescent statereveals that the potentials are virtually independent of thetransistors. Diode 17 is conducting in the reverse direction and diode18 in the forward direction. The voltage drop from the collector oftransistor 2 to the collector of transistor 1 is thus Vbd. Bothtransistors are conducting and, characteristically, have very smallbase-to-emitter voltages, negligible when cornpared to the othervoltages in the circuit. Diodes 13 and 14, which are selected to havebreakdown voltages Vb, larger than Vbd by a desired amount, areconducting in the reverse direction, maintaining the voltage from thecollector of transistor 1 or 2, respectively, to the base of transistor2 or 1, respectively, at the value Vhs. The collector-to-emitter voltageof transistor 1 is Vhs-VM, that of transistor 2, VhS-l-Vbd. Thus thevoltages across resistors 5, 6, 7, 8, 11, 12 are virtually identicalwith those in the simplified equivalent circuit of Fig. 1A. Accordingly,these potentials are independent of the transistor, being dependent onlyupon the voltage of the supply source 4, the magnitudes of the variousresistors, and the diode breakdown voltages.

The next switching transient, like its predecessor, is initiated by theapplication of a pulse to the terminal 19. This pulse adds to thecurrent being carried by diode 17 and subtracts from that being carriedby diode 18, shortly bringing the current in diode 18 to zero andcausing it to open; i.e., to enter its high resistance condition.Thereupon all of the pulse current goes into diode 17 and is coupled tothe base of transistor 2. The collector of tranf sistor 2, now free ofthe constraint of diode 18 regeneratively acts on transistor 1 tocumulatively bring its collector current down while that of transistor 2increases. Still later in the progress of the switching transient thediode 18 breaks down, thus establishing an effective shortcircuitbetween the collector electrodes and terminating the switching transientat a point short of cut-off for the transistor 1 and short of saturationfor the transistor 2.

It follows from the complete symmetry of the circuit of Fig. 1 that thepotential reached by the terminal 19 at one stable end condition of theswitching transient is of the same magnitude as the potential whichterminal 19 reaches at the other extreme condition of the switchingtransient. Hence, if attention be restricted to these two end conditionsthe potential of the terminal 19 does not change. Since any fluctuationsof its potential during the course of the switching transient will havedisappeared before the end condition has been reached, terminal 19 mayfairly be described as a point of effectively fixed potential.

Fig. 2A shows a breakdown diode as described in the Pearson-Sawyerpublication above referred to, poled for conduction inthe forwarddirection from left to right, and

Fig. 2B showsits current-voltage.characteristic, idealized to the extentthat its severalportionsfare exactlyy parallel with the Ycurrent andvoltage axes, instead of very nearly so. Starting from a rest condition,as the voltage across the device is increased only slightly in thepositive direction current commences to ow and ows easily. Starting,however, from the same rest condition, as the voltage is increased inthe opposite direction, substantially no current ows until the voltagehasreached'the breakdown voltage, termed Vdj for thedevice. At thisvalue of voltage, current commences to tlow` in thereverse direction andows easily for any minute further increase in voltage, large` or small.Thus the characteristic comprises three portions: a central'portion inwhich virtually no current ows for voltages lying between zero and Vbd,and throughout which the resistance of the device is substantiallyinnite or at leastvery high, and two branch portions throughout each ofwhich the variational resistance exhibited by the device issubstantially zero or at least very low. One of these branches extendsfrom the central portion at' zero voltage in one direction and the otherbranch extends from the central portion at thevoltage Vbd in theopposite direction.

Evidently, when two devices as shown in Fig. 2A are connected in seriesand in opposing relation as shown in Fig. 3A, vthe voltage-currentcharacteristic ofthe combination is as shown in Fig. 3B. Evidently, too,this combination of two opposed breakdown diodes when connected in thecircuit of Fig. 1 operates to restrict the extreme potential diterencebetween the collector electrodes of thetwo transistors to the magnitudeVbd` in one. sense when the action is to prevent saturationof onetransistor and in the other sense when its action is topreventsaturation ofthe other transistor.

With transistors 1, 2, of conventional construction and havingconventional characteristics, suitable values4 for the circuit constantsare as follows:

An appropriate value for the breakdown voltage of the diodes 17, 18, is6.5 volts. An appropriate valuefor the breakdown voltage of the diodes13, 14, is 8 volts. With the foregoing values for- Rn and R12, thediodes 13, 14, are' the'n maintained continuously'in their broken-downstates. Thus each of them serves the same purpose as would a battery ofthe same voltage connected between the same points. But, because of itssmaller size and electrostaticcapacitance and its greater constancy ofvolt age, vit is `far superior, as a circuit element for maintainingconstant electrode voltage, to a battery. Each of these supplementarybreakdown diodes 13,. 14, may be bypassed by a condenser 15, 16, inaccordance with standard practice for the cross-coupling links of abistable circuit.

In the common circuits in which the switching transient is terminated bysaturation of the transistor, this mode of termination notV only imposesan upper limit to the operating frequencybut italso presentsdiiliculties sometimes encountered in transistors when the emitter isback-biased for substantial periods of time. The present invention, byvirtue of its division of labor between the transistor and the breakdowndiodes not only increases the upper op-v erating frequency limit but, inaddition, it permits each transistor to remain continuously in itsactive or operating state; i. e., its mostv reliable state.

The voltage-current characteristic of Fig. 3B, which is obtained, asdescribed above and as shown in`Fig. 3A, by the connection of the twobreakdown diodes 17, 18, with opposite polarity, may also bel secured bythe combination `6 of asingle .breakdown` diode ,and abatteryy Iasshown-:in Fig. 3C. In ,this casetthe rdiodeis tobefabricatedand selectedto A,have ,at breakdown ,vo1tage,.Vbd ot"` twiceY themagnitudeof.the1breakdownivo1tageof either of'` the diodes` of Fig.`3A,while the battery should haveavoltage of `half thismagnitude; i. e.,

Hencefthe combination ofV Fig. 3Cmay in principle be e'mployed inthecombination ofFig. 1 to'replace `the'comlbination ofFig. 3A. It :surfersfrom the'obvi'ous. disadvantage, however, that .it .utilizes a batterywith'thefattendant hazards .ofhigh capacity, largebulk, short life, etc.Furthermore, therezis with this'imodication no longer ai cohvenientpoint .whichfisl symmetricallyylocatedA with respect to both transistorsfory the application ofl pulses'. Despitethese Adisadvantages `it may,under? some. circumstances;.be found useful. Inasmuch as it'provides'the sameoverall currentvoltage characteristic, it permitscross+coupling by wayof breakdown diodes 13, 14, as in the'case of Fig'.l.

Fig. 4 shows two P-typetransistors 31, 32, whose collector electrodesareconnected by way of supply resistors 35, 36, to the positive terminalof a potential source`34, their emitterelectrodes beingsimilarly-connected by way of resistors 37, 38, to the negative terminalof ther source 34 which may be grounded. The emitter resistorsv 37, 38,are by-passed by. condensers 39, 40. The base electrode of theright-hand transistor 32 is connected by way of a resistor 42` to groundand the base electrode of the left-'hand transistorl 31 is connected: byway of a resistor 41 toa point'of an intermediateV potential such as 8volts. The collector electrode of the right-hand transistor 32 iscross-coupled by way of aA condenserl 4S to the-base electrode oftheleft-hand transistor 31. The collectorelectrode of the left-handvtransistor 31 is crosscoupled by way of` a breakdown diode 44, shuntedby a condenser 46, to the base electrode of the right-handftransistork32'. By reason of the asymmetry in the cross*- coupling links thiscircuit is monostable, and itsone stable condition is that in which` theleft-hand transistor rests at or near cut-off while the right-handtransistor' rests at or near saturation. Pulses, originating for examplein a source 50, may be applied by way of a condenser 51 and an ordinarycrystal diode 52 tothe base electrodeV of theleft-hand transistor 31.Application of any s'uch pulse initiatesr a switchingv transient,causing the conduction conditions of the two transistorsto be reversedfor a time. During the time whenA transistor 31 is conducting more'heavily, condenser 45 is charged from resistor 36 r and battery 34'. Thebase of'` transistor 31 cannot be maintained` at a suicientl'y highpotential. to perpetuate the high conduction ofy transistor 31,- whichis after a time regeneratively returned to its'initial state of lowconduction. Suitable magnitudes for the circuit constants' are asfollows:

R35=l0,000 o'hms R36: 15,000 ohms R37=R39=3,000 ohms R41=15,000`ohmsR42: 100,000 ohms C9=400 ,U4/. CMI- [LMfL vota (ein N-typetransistors-the polarities of the voltage sources and of these breakdowndiodes should be reversed).`

From the foregoing description of the operation of the circuit of Fig. 1it will be apparent that, in Fig. 4, when the potential of the collectorelectrode of the left-hand transistor 31 exceeds 15 volts the lowerdiode 48 breaks down and when, on the other hand, the potential of thesame collector electrode is less than 10 volts the upper diode 47 breaksdown. The breakdown of one of these -diodes prevents the left-handtransistor 31 from reaching its saturation condition and the breakdownof the other diode prevents the left-hand transistor from reaching itsIcut-ofi? condition and so, by virtue of the cross-link diode 44prevents the right-hand diode 32 from reaching its saturation condition.Thus the transistors 31, 32, are fheld at potential short of saturationpotentials in turn and so the sluggishness which attaches to thesaturation condition of a transistor is obviated.

The advantage of restricting the voltage excursions of 'a transistortrigger circuit to magnitudes short of saturation apply also to circuitsemploying a single transistor. Fig. 5 shows a well-known example of thisclass of circuit, namely, a blocking oscillator including a transistor61, energized by a source 62, in which regenerative feedback is securedthrough inductive coupling, as by a transformer 63 which couples thecollector electrode to the emitter electrode. The operation of such acircuit is too well known to require description. Minor changes of awell-known character cause the circuit to be free running, in which casethe trigger pulse source 65 may be omitted lif desired, though it may onthe other hand be retained as a source of synchronizing pulses.

In accordance with the invention a breakdown diode 68 is connectedbetween two suitable points; e.g., across the primary winding of thecoupling transformer 63. It is poled in the direction shown for a P-typetransistor 61. For an N-type transistor its polarity, as well as that ofthe power supply source 62 and the bias source 64, should be reversed.The diode 68 operates in the manner described above to limit thecollector voltage swing in one direction to a value short of saturation.Thus, it acts not only to retain the transistor 61 in its linearamplifying range throughout much of each single oscillation, and so toincrease the upper operating frequency limit, but it acts at the sametime, and by virtue of the same effect, to render the circuitinsensitive to the idiosyncracies of individual transistors and so ineiect to make the transistors which may be employed as the activeelements of the circuit more nearly interchangeable.

Ordinary diodes are frequently used in blocking oscillators to dischargethe magnetizing inductance of the transformer and prevent largeoscillations after the principal pulse. Diode 68, by virtue of its lowresistance in the forward direction, performs this function as it standsin addition to its principal function of preventing saturation in thetransistor.

What is claimed is:

1. A non-saturatng trigger circuit which comprises a transistor having asemiconductive body, an emitter electrode, a base electrode and acollector electrode, a source of potential for energizing saidelectrodes, one of said electrodes serving as an input electrode,another of said electrodes serving as an output electrode, and `thethird of said electrodes serving as a common electrode, an amplifyingsecond transistor having a structure and characteristics like those ofsaid first-named transistor coupling the output electrode to the inputelectrode for providing regenerative feedback to promote instability,whereby voltage excursions of said output electrode are normally limitedon one hand by a cut-off condition and on the other hand by a saturationcondition, a thresholdresponsive device characterized by a first lowresistance condition for voltages across it which are of one polarityand of any magnitude, by a second low resistance condition for voltagesacross it which are of the opposite polarity and of magnitudes exceedinga preassigned threshold, and by an intermediate high resistancecondition, said device being connected between said output electrode anda point of effectively fixed potential lying midway between thepotentials of the collector electrodes of said transistors, said devicefurther being poled and adjusted to be in its high resistance conditionthroughout the greater part of a switching transient oscillation of saidcircuit and to be driven into its second low resistance condition foroutput electrode voltages in the neighborhood of, but short of,saturation voltages, a second threshold responsive device havingcharacteristics like those of said first-named threshold responsivedevice connected in series opposition with said first-named thresholdresponsive device.

2. A nonsaturating trigger circuit which comprises a transistor having asemiconductive body, an emitter electrode, a base electrode and acollector electrode, a source of potential for energizing saidelectrodes, one of said electrodes serving as an input electrode,another of said electrodes serving as an output electrode and the thirdof said electrodes serving as a common electrode, an amplifying secondtransistor having a structure and characteristics like those of saidfirst-named transistor coupling the output electrode to the inputelectrode for providing regenerative feedback to promote instability,whereby voltage excursions of said output electrode are normally limitedon one hand by a cutoff condition and on the other hand by a saturationcondition, a threshold responsive device characterized by a tirst lowresistance condition for voltages across it whichare of one polarity andof any magnitude, by a second low resistance condition for voltagesacross it which are of the opposite polarity and of magnitudes exceedinga preassigned threshold, and by an intermediate high resistancecondition, said device being connected between said output electrode andone terminal of said operating potential source, said device furtherbeing poled and adjusted to be in its high resistance conditionthroughout the greater part of a switching transient of oscillation ofsaid circuit and thereby driven into its second level low resistancecondition for output electrode voltages in the neighborhod of, but shortof, saturation voltages, and a second thresholdresponsive device, havingcharacteristics like those of said lirst-named threshold-responsivedevice, interconnecting the collector electrode of the first-namedtransistor with the opposite terminal of said potential source and poledin the same sense with respect to the potential source terminal as saidfirst-named threshold-responsive device.

3. A nonsaturating trigger circuit which comprises a pair oftransistors, each having an emitter electrode, a collector electrode anda base electrode, a potential source for supplying energy to saidelectrodes, impedance elements individually connecting said emitterelectrodes to a point of one iixed potential, impedance elementsindividually connecting said collector electrodes to a point of anotherfixed potential, a cross-coupling path extending from the collectorelectrode of each transistor to the base electrode of the othertransistor, means including a threshold-responsive device connected toan electrode cf at least one of said transistors for restricting theextreme potential excursion of said one electrode to a value less thanits normal, operation-limiting, value and, in combination therewith, asubstantial non-zero constant-voltage element connected in series in atleast one of said crosscoupling paths.

4. A non-saturating trigger circuit which comprises a pair oftransistors, each having an emitter electrode, a collector electrode anda base electrode, a potential source for supplying energy to saidelectrodes, impedance elements individually connecting said emitterelectrodes to a point of one fixed potential, impedance elementsindividually connecting saidl collector electrodes to a point of anotherxed potential, a cross-coupling path extending from the collectorelectrode of each transistor to the base electrode of the othertransistor, means including a threshold-responsive devices connected toan electrode of at least one of said transistors for restricting theextreme potential excursion of said one electrode to a value less thanits normal, operation-limiting, value and, in combination therewith, asubstantial non-zero constant-voltage element connected in series ineach of said cross-coupling paths.

5. In combination with apparatus as defined in claim 3, a condenserconnected in shunt with said constant-voltage element.

6. Apparatus as defined in claim 3 wherein said constant-voltage elementcomprises a threshold-responsive device of which the threshold voltageis adjusted to a value intermediate between said potential sourcevoltage and a voltage of polarity like to that of said potential sourceand substantially removed from zero.

7. Apparatus as dened in claim 6 wherein said threshold-responsivedevice comprises a semiconductor PN junction alloy diode.

8. Apparatus as defined in claim 6 wherein said threshold-responsivedevice is a silicon PN junction alloy diode.

References Cited in the le of this patent UNITED STATES PATENTS

